Maternal vaginal microbes provide the natural seeding to the newborn microbiota (1). Whether vaginal microbes can reach the placenta and the fetus before labor initiates, still unclear (2, 3), but mode of delivery overwhelms any other possible previous signal, and C-section-born babies are microbiologically different from vaginally born infants (1). The maternal vaginal (4) and intestinal (5) microbiota change during the third trimester of pregnancy, but the significance of these changes for the fitness of the baby has not been understood. Early interaction with indigenous microbes is essential for healthy immunological and metabolic programming, and contact with bacterial populations in the vagina during birth marks the beginning of eventual massive bacterial colonization of the newborn's mucosal surfaces. Mucosal immunity is strongly influenced by the microbiota (6), which in the gut mucosa, is subject to continuous surveillance by M cells—from the Peyer's patches of the gut-associated lymphoid tissue (GALT)—for processing by local dendritic cells and subsequently modulate CD4+ to produce Tregs and induce tolerance.
The “education” of the immune system by the microbiota starts at the very first microbial exposure and pioneer bacteria probably play a determinant role. Few studies have focused on the development of the intestinal microbiota and immunity (7-10), but the assembly in multiple body sites, the fate of vaginal lactic acid bacterial populations, and the ecological dynamics between maternal and infant communities that interact are not known. Despite the vast descriptive knowledge about the microbiota of American adults generated by the HMP project (NIH), little is known about developmental aspects of the microbiota.
The incidence of immune, inflammatory, and metabolic disorders is increasing in industrialized countries (11, 12). Early life events and aberrant microbial colonization has been associated with these diseases (12, 13). Epidemiological associations show C-section delivery increased risk of asthma and allergies (14-21), intestinal inflammatory conditions (19, 22-24). Obesity and diabetes (type I) is also increased by C-section birthing (19, 25-29) and by antibiotic consumption (19, 27-30) even as early as during gestation (25).
The phenomenon of decreased oral and gut microbial diversity in the first days after birth, previously reported in mice (36), is of unknown functional significance, but might be reflecting the selective effect of milk on the gut microbiota. Bacteroides, Clostridiales and Bifidobacterium are bacteria that are enriched in the gut during the first weeks of life, during strict lactation.
Cesarean section birthing without maternal membrane rupture, as in the case of scheduled Cesarean, impedes the seeding of the babies with vaginal microbes. C-section is medically indicated in 13-15% of the births, saving many lives of mothers and babies. However, scheduled C-section is becoming the standard of birthing in many countries of the world, with over 50% of births in Brazil, Dominican Republic, and Iran and many other countries approaching these rates (Health at a Glance 2011: OECD Indicators; WHO Global Health Observatory; (31); (37)). C-section birthing is associated with short health risks for the mother and baby and with long term health risks for the babies, including celiac disease (23, 24), asthma (16, 20, 21, 38), type 1 diabetes (39, 40), and obesity (32, 41, 42).